Chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various components of the sample—which differ based upon differences among partition coefficients of the elements—cause the sample to be separated into are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Often, the sample is first obtained using a sampling device, which subsequently transfers the sample to the chromatograph. One means of obtaining a sample and introducing it into a chromatographic column is known as “headspace sampling.” In conventional headspace sampling, sample material is sealed in a vial and subjected to constant temperature conditions for a specified time. Analyte concentrations in the vial gas phase should reach equilibrium with the liquid and/or solid phases during this thermostatting time. The vial is subsequently pressurized with carrier gas to a level greater than the “natural” internal pressure resulting from thermostatting and equilibration. Then the pressurized vial is connected to the chromatographic column in such a way as to allow for the transfer of a portion of the vial gas phase into the column for a short period of time. An example of such a sampling device is disclosed in U.S. Pat. No. 4,484,483 to Riegger et. al. An example of a chromatographic system employing such a sampling device is disclosed in U.S. Pat. No. 5,711,786 to Hinshaw, which describes using a chromatographic injector between the vial and the chromatographic column.
Often, it is desired to include a known quantity of a known substance—often referred to as an “internal standard”—in the analysis. This internal standard, which contains one or more compounds that are known not to be present in the sample but that are of a similar concentration and chemistry as the sample compounds, can provide reference peaks during the chromatographic analysis to aid peak identification or to improve quantitative precision. For example, in the final chromatographic analysis, the peaks from these standard gas compounds can be identified, quantified, and used to make a ratiometric correction to the quantitative results from the analyte compounds. This technique compensates for instrumental variations that may affect the analytical results, as both the standard gas compound and each analyte are subjected to the same variations, and thus, their relative responses provide a more valid quantitative measure of the amount of analyte present.
For a number of reasons, it can be valuable to introduce the internal standard into the sample vessel itself, prior to the extraction of headspace vapor therefrom. For example, a headspace sampler is typically used to test a number of large number vials in sequence, which are usually held in a rotating carousel or moving rack of some sort. When dealing with a large number of vials in a given autosampling sequence, it is possible for one or more vials to have a leak, which obviously leads to erroneous analytical data. Accordingly, it is advantageous to have the internal standard already present in these vials prior to headspace extraction so that problems with vial integrity may be detected.
Similarly, it is often desired to pre-concentrate the analytes in the sample, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. Accordingly, as disclosed in U.S. Pat. Nos. 5,792,423 and 6,395,560 to Markelov, these systems will typically include some kind of “trap” for this purpose, which retains the analytes as they are carried through the trap, and which are later released from the trap, usually by heating, and swept into the chromatographic column. One example is adsorbent traps, which adsorb the analytes and then subsequently desorb those analytes into the chromatographic column, such as the arrangements disclosed in U.S. Pat. No. 5,932,482 to Markelov and U.S. Pat. No. 6,652,625 to Tipler. Accordingly, it is often advantageous to mix the internal standard in with the sample prior to entry into the trap in order to test the trap's efficiency.
A number of systems have been proposed for introducing a standard into a sample vessel. Examples of such arrangements are disclosed in U.S. Pat. No. 6,706,245 to Neal et al. and U.S. Pat. No. 5,998,217 to Rao et al. Each of these systems, however, is somewhat complex, involving multiple transfer lines and complex needle assemblies.
What is desired, therefore, is a system and method for introducing a standard gas into a sample vessel that does not require complicated flow path or needle assemblies. What is further desired is a method for introducing a standard gas into a sample vessel that does not require additional time to perform additional steps.